1. Field of the Invention
The present invention relates to a liquid crystal display device which is back-illuminated by means of LED's.
2. Prior Art
A liquid crystal display (LCD) device is used in various different types of devices and instruments such as OA (office automation) equipment, clocks, blood pressure gauges, hot water supply devices, etc. The display element (or display) is illuminated from the inside so that the visibility of the numbers, characters, etc. displayed thereon is improved.
In small battery-driven liquid crystal display devices, a reflective illumination method is generally employed so as to reduce power consumption. In this method, a light bulb (incandescent light bulb) is installed inside the LCD device, and the bulb is lit only when it required by the operator. The reason for this occasional use of the light bulb is that, though it depends on the capacity of the battery installed in the device, the power available is as small as 60 mW to 100 mW, and it is not possible to obtain a constant lighting by the light bulb.
Recently, therefore, illumination of the display is accomplished by a back-lighting system that uses LED's (light-emitting diodes) because they are low in power consumption.
LED's have a much smaller power consumption (20 mW per unit) than ordinary bulbs and also have a high brightness and a long useful life. In addition, LED's can emit various different colors of light such as green, yellow, etc. in addition to red; and they are easy to obtain. Accordingly, LED's are ideal for use as an illuminating light source in the liquid crystal display devices.
However, with LED's alone, it is difficult to illuminate a broad area of the display uniformly, since the directionality of the emission output of the LED's is high.
Accordingly, as shown in FIG. 12, a flat panel type light-emitting body which is obtained from a light-conducting plate 1 and LED 2 is used as an illuminating device for a liquid crystal panel 3.
In this flat panel type light-emitting body, the light-conducting plate 1 is made of a transparent resin which is superior in terms of light transmissibility (e.g., an acrylic resin, etc.) and formed into a wedge shape when viewed from the side so that the back surface 1a of the light-conducting plate 1 is inclined. Light 4 emitted from the LED 2 enters into the interior of the light-conducting plate 1 from the thick end surface 5 and is then reflected at the back surface 1a of the light-conducting plate 1 so that light is emitted out from the top surface 6. Thus, a uniform surface illumination is obtained, and the entire surface of the liquid crystal display panel 3 is uniformly illuminated.
Compared to other flat panel type light-emitting bodies that use electroluminescence, optical fibers, etc., this flat panel type light-emitting body has advantages. It is simple in structure and low in the manufacturing cost.
Since the LED 2 has a small light emission area, a diode chip 8 is molded in a transparent resin 7 such as an acrylic resin, etc. so that the light emitted from the diode chip 8 is spread by a convex lens part 7a formed at the tip of the transparent resin 7. With this structure, the light is easier to see.
In the above light-conducting plate 1, the inclined back surface 1a forms a reflective surface 9 which is obtained by the vacuum evaporation of aluminum or bonding of a reflective sheet, etc..
However, the following problem arises in the conventional liquid crystal display devices that use a light-conducting plate 1 and LED 2. Brightness irregularity tends to occur; and as a result, the entire display of the liquid crystal display panel 3 is not uniformly illuminated.
More specifically, the LED 2 is installed so that its optical axis L is roughly parallel to the surface 6 of the light-conducting plate 1. A portion 4a of the light entering the light-conducting plate 1 from the LED 2 is reflected by the reflective surface 9 and is emitted to the outside from the surface 6 (this light is called directly reflected light), while another portion 4b of the light is first completely reflected by the surface 6 of the light-conducting plate 1 and then reflected by the reflective surface 9 and emitted from the surface 6 (this light is called completely reflected light). Accordingly, if the reflective surface 9 is divided into, for example, three areas A, B and C along the direction of the optical axis (the area closest to the LED 2 is A, the next closest area B, and the area furthest from the LED 2 C), the quantities of directly reflected light 4a and the completely reflected light 4b per unit area vary depending upon the area involved. As a result, brightness irregularity occurs. The areas A and C are brighter, and the area B is darker.
In compact liquid crystal display devices, the manufacturing cost, brightness, size and weight are critical, and devices which are low in manufacturing cost, size and weight and which provide a uniformly bright illumination in strong demand. However, in the devices which use the light-conducting plate as described above, the weight tends to be large because of the light-conducting plate. Thus, there are limitations to the weight reduction.